1. Field of the Invention
The present invention relates to a magnetic shield apparatus applied to a magnetic resonance imaging system, or the like using a magnetic field to obtain medical information of a patient.
2. Description of the Related Art
A static field generating device for generating a static magnetic field is used in a conventional magnetic resonance imaging apparatus. In general, the static field generating device must generate a high, uniform magnetic field in order to realize a high quality image. A superconducting magnet unit using a superconductor has been recently used as a static field generating device capable of generating a high magnetic field.
A conventional static field generating device using a superconductor comprises magnet unit 2, as shown in FIG. 1. Magnet unit 2 includes double cylindrical casing 4. Casing 4 includes inner and outer cylinders 6 and 8. Both ends of the space between inner and outer cylinders 6 and 8 are closed by end plates 10, respectively. Both ends of inner cylinder 6 are open. Columnar cavity 14 is defined by the inner circumferential surface of inner cylinder 6 of casing 4. Superconducting coil 16 is arranged with coolant 18 such as liquid helium in casing 4, i.e., between inner and outer cylinders 6 and 8. A current is supplied to superconducting coil 16 through power supply leads (not shown). Therefore, magnet unit 2 is excited in a permanent current mode.
With this arrangement, a high, uniform static magnetic field can be generated along the axial direction of columnar cavity 14.
However, magnetic leakage cannot be neglected. More specifically, as a magnetic field in cavity 14 is increased, the magnitude and area of magnetic leakage are increased. For this reason, the magnetic leakage adversely affects information processing units and other medical equipments which are mounted around the static field generating unit. For example, the magnetic leakage causes distortion of an image from an image pickup tube of an X-ray TV system and an image from an image intensifier, or color misregistration in a color TV monitor. For this reason, installation conditions of the magnetic resonance imaging apparatus are undesirably complicated due to the influence of magnetic leakage.
In order to solve the above problems, the outer surface of the magnet unit has been covered with a magnet shield comprising a magnetic body such as iron. More specifically, a magnetic shield having substantially the same shape as a casing is integrally mounted on the outer surface of the casing. Then, the influence of magnetic leakage can be reduced.
However, since the weight of the magnet unit is about 3.0 tons or more, it is difficult to transport the magnet unit to a facility such as a hospital. When the magnetic shield having substantially the same weight as the magnet unit is additionally mounted on the magnet unit, the weight and size of the entire static field generating device are increased. Therefore, it is more difficult to transport the magnet unit to a facility.